feat(worker): let Claude set the cheapest model per generated task via MCP

AddTask, planning CreateChildTask, and SuggestImprovement now accept an
optional alias-validated model (haiku/sonnet/opus; blank = inherit) so the
model is chosen at creation time instead of a follow-up set_task_config call.
The planning, system, and improvement prompts instruct Claude to pick the
cheapest capable model (haiku < sonnet < opus).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/ClaudeDo.Data/Repositories/TaskRepository.cs

@@ -197,6 +197,7 @@ public sealed class TaskRepository
         string? description,
         string? commitType,
         string? createdBy = null,
+        string? model = null,
         CancellationToken ct = default)
     {
         // AsNoTracking: SetPlanningStartedAsync mutates via ExecuteUpdate which
@@ -223,6 +224,7 @@ public sealed class TaskRepository
             ParentTaskId = parentId,
             SortOrder = (maxSort ?? -1) + 1,
             CreatedBy = createdBy,
+            Model = ModelRegistry.NormalizeAlias(model),
         };
         _context.Tasks.Add(child);
         await _context.SaveChangesAsync(ct);